Field of the Invention
The present disclosure relates to an electrophotographic photosensitive member, and a process cartridge and an electrophotographic apparatus each including the electrophotographic photosensitive member.
Description of the Related Art
Multilayer photosensitive layers including a charge generating layer containing a charge generating material and a charge transport layer containing a charge transporting material are the mainstream of the photosensitive layer of electrophotographic photosensitive members. Multilayer photosensitive layers have advantages of, for example, being highly sensitive and allowing a variety of material design.
Phthalocyanine pigments, which are superior as photoconductor and are highly sensitive to light in a wide range of wavelengths, are used as a charge generating material of the electrophotographic photosensitive member of electrophotographic apparatuses using a semiconductor laser capable of oscillation in a wide range of wavelengths as an image exposure device. It has been known that phthalocyanine pigments exhibit various electrical properties, depending on the crystal form thereof and also on the manufacturing process (which is varied in treating method performed by, for example, UV irradiation, pulverization, or using solvent, or in synthesizing method) even if the crystal form is the same.
When a photosensitive material is used in an electrophotographic process in practice, it is desirable that the S/N ratio of the difference (latent image contrast) between the charged potential of the non-image area and the exposure potential of the image area be high. The term “S/N ratio” used herein refers to the ratio of the difference between charged potential and exposure potential to the decrease in charged potential caused by various reasons including dark decay and repeated use of the photosensitive member or to the increase in exposure potential caused by various reasons including uneven thickness of the charge generating layer and repeated use of the photosensitive member. By increasing the S/N ratio to stabilize the latent image contrast, both the difference (development contrast) between development potential and exposure potential and the difference (Vback) between charged potential and development potential are stabilized. When the development contrast is stable, the amount of toner in the image area becomes stable. Also, when the Vback value is stable, fogging over the non-image area (phenomenon in which toner is developed in an area where charged potential is reduced). Thus, increasing the S/N ratio of latent image contrast leads to improved image quality.
In view of recent demands for high image quality, high speed output and long life of electrophotographic images, it is particularly desired to suppress the increase of dark decay to keep the S/N ratio of the latent image contrast high after repeated use. An increased dark decay results in a reduced charged potential for development and a reduced S/N ratio of the latent image contrast. Consequently, the Vback value becomes unstable, causing fogging. From the viewpoint of reducing dark decay, a method has been being studied for forming a charge generating layer to a small thickness.
However, the amount of light that a thin charge generating layer can absorb is small. Accordingly, the ratio of energy of light absorbed by the photosensitive layer to the total energy of exposure light, that is, light absorptance of the photosensitive layer, decreases, and consequently, the sensitivity decreases and becomes unstable. This sometimes causes exposure potential to increase and become unstable, reducing the S/N ratio of the latent image contrast. Accordingly, some approaches have been proposed for improving the performance of the phthalocyanine pigment itself by increasing the ratio of the number of photocarriers generated from the charge generating material to the number of photons absorbed by the photosensitive layer, that is, quantum efficiency (Japanese Patent Laid-Open Nos. 2006-72304, 9-138516, and 7-319188).
Japanese Patent Laid-Open No. 2006-72304 discloses an electrophotographic photosensitive member using a technique in which a mixture of a phthalocyanine pigment, an organic electron accepter, and a specific solvent is pulverized in a wet process so that the organic electron accepter is taken into the surfaces of the phthalocyanine pigment particles and/or the vicinities of the surfaces while the crystal form of the phthalocyanine pigment is changed. According to this prior art document, this technique can sufficiently impart a chargeability, a sensitivity to light, and a low dark decay to the electrophotographic photosensitive member, thus reducing image defects, such as fogging and ghosting.
Japanese Patent Laid-Open No. 9-138516 discloses an electrophotographic photosensitive member containing: a phthalocyanine compound having a particle size distribution in which particles with a particle size in the range of 0.1 μm to less than 0.5 μm account for 60% of the total volume of the compound; and an organic compound having a specific structure and capable of acting as an acceptor. According to this prior art document, this technique reduces residual potential to impart high sensitivity to the photosensitive member while improving the dispersibility of phthalocyanine pigment.
Japanese Patent Laid-Open No. 7-319188 discloses an electrophotographic photosensitive member including a photosensitive layer containing a binder resin and a titanyl phthalocyanine pigment (oxytitanium phthalocyanine) dispersed in the binder resin. This titanyl phthalocyanine pigment exhibits a CuKα X-ray diffraction spectrum having the strongest peak at a Bragg angle 2θ of 26.3°±0.2° with a half width (full width at half maximum) of 0.4° or less. According to this prior art document, the charged potential of this electrophotographic photosensitive member is not much reduced even by repeated use, and thus the electrophotographic photosensitive member exhibits good electrical properties. The half width depends on the manufacturing conditions, such as the time for pulverization or dispersion, the size and specific gravity of the pulverization or dispersion media such as beads or balls, and the rotational speed of the pulverization or dispersion mill such as a ball mill. This prior art document explains that this is because the crystal lattice of the titanyl phthalocyanine can be irregularly distorted by the stress placed thereon by pulverization or dispersion.